The present invention relates to lift cranes with a pivotal boom having a load hoist line extending from the boom, and particularly to a lift crane with an assist structure for helping to raise the boom during a set-up operation.
Lift cranes typically include a carbody; ground engaging members elevating the carbody off the ground; a rotating bed rotatably connected to the carbody such that the rotating bed can swing with respect to the ground engaging members; and a boom pivotally mounted on the rotating bed, with a load hoist line extending therefrom. Lift cranes with a pivotal boom also include a boom hoist mechanism that can be used to change the angle of the boom relative to the rotating bed during crane operation. For mobile lift cranes, there are different types of moveable ground engaging members, most notably tires for truck mounted cranes, and crawlers. Typically lift cranes include a counterweight to help balance the crane when the crane raises the boom or lifts a load. Also, lift cranes are typically built with booms made of multiple boom sections, some of different lengths, to construct booms of different lengths. In this way a crane can be assembled with a different length boom based on the lift to be performed, with longer booms being used when the lift to be performed involves a greater height or longer reach.
Lift cranes are typically designed based on the largest load that they can lift, and also have to be designed taking into account the moment created by the load and the boom when the crane lifts the load at various boom angles and lengths of boom. Typically a crane manufacturer will provide load charts for each crane it sells, showing the maximum loads that can be lifted at different boom angles for each boom length. These load charts take into account the structural capability and stability of the crane design. Structural capability relates to the fact that the crane components can withstand the loads on the individual parts that are generated as a lift is performed. For example, a slewing ring has to be built out of pieces with enough strength so that when a crane lifts a load, the forces on each component of the slewing ring, such as the rollers, can be withstood. Likewise, the boom has to be built so that it does not buckle when all of the compressive forces act on the individual members of the boom. For many components the structural capability is concerned with both direct forces and moment forces, and has to take into account the fact that the crane can swing or travel with a load on the hook. Stability, on the other hand, is mostly concerned with the crane as a whole being able to stay upright during crane lifting operations. If too large of a load is lifted at a low boom angle, the moment created by the load and the outstretched boom measured from the front fulcrum (typically the furthest point where the crane's crawlers engage the ground) might cause the crane to tip over. Adding counterweight increases the stability of the crane, but then also requires the structural capacity of the crane to be increased.
In addition to the maximum load that can be lifted, a lift crane has a limit to the weight and length of the boom that can be raised off the ground by the crane during crane set-up. Booms that can withstand greater compression, and thus increase the maximum lift capacity of the crane, usually require greater cross sections and thicker members. These features, however, increase the weight per unit length of the boom. When a crane is trying to lift the boom off the ground during a set-up operation, the boom is at a horizontal boom angle, and the moment created by just the weight of the boom and items fixed on the boom top are tremendous.
Most crane designs are balanced such that both the structural capability and the cranes stability limit the maximum length of boom that can be raised from the ground. In practice, it is common to have slightly more structural capability than stability, i.e., stability generally governs the maximum boom length and weight that can be raised.
Crane users would like to be able to raise longer booms to achieve greater reach, or booms with more weight to achieve greater capacity. In some cases users want both more length and capacity. In times past it was possible to use a longer/heavier boom than the crane could lift by itself by having an assist crane on site to assist with boom raising and lowering when the crane is assembled and disassembled. However, if the boom needs to be lowered in an emergency, and the assist crane is not available, there is no easy way to lower the boom to the ground without causing the crane to tip.
Crane manufactures have responded by providing features on their cranes that allow the crane to raise a longer boom than might be otherwise possible. For example the Liebherr LR1600/2 model crane is equipped with an added pair of raising supports to one side of the carbody. These increase the fulcrum and thus provide greater boom raising stability. However, because the raising supports are on the carbody, the entire crane's structural system (all structural components) must be increased to allow a longer/heavier boom to be raised.
Thus there is a need for a way to supplement the stability of a crane in such a way that the crane can raise a longer and/or heavier boom during the crane set-up operation without the need to increase the structural capacity of the crane, and which does not require that an assist crane be readily available.